YANGON, Myanmar (AP) — Police in Myanmar said they were investigating the head of a mosque and a Muslim teacher for possible negligence after a pre-dawn fire swept a religious dormitory Tuesday, killing 13 children in a blaze that raised new concern over sectarian tensions that have plagued the country since anti-Muslim violence hit the nation’s heartland last month.

Authorities blamed the fire on an electrical short circuit and deployed riot police to maintain calm. But some Muslims remained suspicious, saying it was set intentionally.

Myanmar has been on edge after sectarian unrest between Buddhists and Muslims erupted in the central city of Meikhtila in March, killing dozens of people and displacing more than 12,000, mostly Muslims. The violence has since spread to several other towns where extremist Buddhist mobs have torched or ransacked mosques and Muslim-owned property.

Police officer Thet Lwin said about 75 children lived in the torched compound in eastern Yangon — which encompasses a mosque, a school and a dormitory — and most were able to escape by running out of a door rescue workers knocked open. Security bars blocked most of the building’s windows, which were stained by black smoke hours after firefighters put out the flames.

Mosque member Soe Myint said most of the children, who had been sent to the religious boarding school by their parents, were sleeping on the ground floor when the blaze began and were able to flee.

But 16 were sleeping in a small loft and were trapped when the stairs to it caught fire. Three boys jumped to safety and the rest died, he said.

Soe Myint, who said he helped carry the dead out of the mosque, said he did not believe the fire was caused by a short circuit and urged authorities to launch a thorough investigation.

“The whole mosque smelled of diesel,” he said. “We don’t use diesel at the school.”

Yangon Division Chief Minister Myint Swe told reporters late Tuesday that police discovered a diesel container underneath a staircase. He said the fuel was normally used to power a mosque generator when the electricity is out.

Hla Myint, whose 15-year-old nephew died in the blaze, waited in a crowd outside Yangon General Hospital, where the dead were taken. Two trucks of riot police were parked nearby.

“We sent him to school only yesterday and today he is dead,” she said. “We are so sad we can’t express it.”

Later Tuesday, several thousand mourners gathered at a cemetery on the outskirts of Yangon for a group burial. The charred bodies of the children were wrapped in white cloth before being lowered into the ground as women wept nearby.

U.S. Ambassador Derek Mitchell issued a statement saying he was “deeply saddened” by the deaths. He also called on the government to “conduct a thorough and transparent investigation into the cause.”

Authorities are investigating the head of the mosque and a teacher, but no arrests had been made, said city Police Chief Win Naing. “As the two people in charge, they are responsible for this and we have to take action against them,” he said.

Win Naing said the fire started in a voltage regulator under the stairs that led to the sleeping loft and that firefighters had to break two locks on the door of the mosque to rescue the survivors. He ruled out arson, saying that three police were guarding the mosque and saw no one approach the building before the fire started.

Speaking in the immediately hours after the blaze, Thet Lwin, the policeman, blamed the fire on an electrical short circuit “and not due to any criminal activity.”

Every time he mentioned the word “electrical short,” though, angry Muslims shouted and began banging on vehicles with their fists.

He also appealed to journalists for help. “We need the media’s support in Yangon. Please don’t report that there is conflict in Yangon. We’re here to stop conflict,” he said.

Security forces and three trucks of riot police blocked off roads around the scarred building in Yangon as a crowd of 200 onlookers, mostly Muslims, gathered.

Zaw Min Htun, a member of a local Muslim youth organization, said he raced to the mosque after hearing it was on fire. He said he entered the charred building and also smelled fuel.

“Muslims are very angry,” he said, calling on authorities to investigate. “The children are innocent. … Someone burned the mosque.”

The recent upsurge in sectarian unrest in Myanmar has cast a shadow over President Thein Sein’s administration as it struggles to make democratic changes after a half century of military rule. His government has warned that the violence could threaten the reform process.

Hundreds of people were killed last year and more than 100,000 made homeless in violence in western Myanmar between ethnic Rakhine Buddhists and Rohingya Muslims. On March 20, unrest hit the central town of Meikhtila for several days and then spread to several villages farther south, near the capital, Naypyitaw.

The violence has spooked people in Yangon, where late last month false rumors swirled of mosque burnings and authorities told some shops to close as a precautionary measure. Yangon is about 550 kilometers (340 miles) south of Meikhtila.